This invention relates generally to a two-stage process for the reductive stripping of uranium from wet-process phosphoric acid, and more particularly, to providing the ferrous ions necessary for the reductive strip in the first stage without contaminating the uranium-enhanced reductive strip solution supplied to the second stage with undesirable impurity elements such as excess iron.
Phosphates for use in the fertilizer industry are obtained by mining phosphate-bearing rock and then converting the rock to an agriculturally useful fertilizer product by one of several alternative processes. In one of these processes, the "wet process", the mined rock is digested in sulfuric acid to produce dilute phosphoric acid and then concentrated to product grade phosphoric acid by evaporation. The phosphate bearing rock may contain several kinds of metallic elements in relatively low concentration which are also dissolved into the phosphoric acid, and these metallic elements may be removed from the acid either as valuable by-products or as undesirable contaminants.
Depending upon the specific type of phosphate rock, uranium may be present in the dilute wet-process phosphoric acid in an amount sufficient to warrant recovery of the uranium for use in the nuclear industry. Several processes for recovering uranium from wet process phosphoric acid are known, and in one such process disclosed in U.S. Pat. No. 3,711,591 the chemical properties of various oxidation states of the uranium ion are utilized to allow extraction of the uranium values by contacting the phosphoric acid with particular extractants dissolved in an organic solvent. This process functions more efficiently if the uranium content in the input feed is initially concentrated in a first stage, wherein the extractant in the organic solvent extracts uranium values from the wet-process phosphoric acid having a very low uranium concentration, and then transfers the uranium values to a uranium-enhanced reductive strip solution used as input to the second stage, by a reductive strip wherein ferrous ions reduce the oxidation state of the uranyl ions. The reduced uranous ions then transfer from the organic-based phase to the aqueous uranium-enhanced reductive strip solution. In existing processes of this type, ferrous ions for use in the first stage reductive strip are provided by treating a portion of the raffinate phosphoric acid from the extraction by the addition of iron metal, which reduces the ferric ions in the raffinate to the ferrous state.
Reduction of the ferric ions in the raffinate by the addition of metallic iron has the significant disadvantage of increasing the concentration of iron in the uranium-enhanced reductive strip solution which serves as the input to the second stage of the recovery process. The iron is extracted together with the uranium in the second stage, and the extracted iron contaminates the concentrated uranium oxide product, acting to complicate the subsequent purification of the uranium oxide product into a form usable in the nuclear industry. To overcome this problem of increased iron concentrations in the uranium-enhanced reductive strip solution, processes have been developed wherein the iron is removed prior to the second stage extraction by precipitation, or the second-stage extraction process itself may be modified to avoid extraction of the iron. In either approach to removing the iron, there are economic disadvantages in that costly chemicals or expensive capital equipment are required.
Accordingly, there has been a need for an alternative approach to providing ferrous ions to be used in the first stage reductive strip which eliminates the disadvantages produced by the addition of iron metal or other chemical reducing agent to the raffinate phosphoric acid. Preferably, the improved process would utilize the same basic process approach as the existing two-stage reductive stripping process for recovering uranium from wet process phosphoric acid, which has been proved to be reasonably efficient. The present invention fulfills this need, and further provides related advantages.